Fix SWRC for one-layered soils

[dschlaep]

• fix swrc_conversion(): repeat one-layer swrcp only after we have checked for agreement with soil variables
• additional unit tests for swrc_conversion()

[codecov[bot]]

Codecov Report

Merging #232 (a0c4066) into release/devel_v6.0.0 (9f21042) will increase coverage by 0.07%. The diff coverage is 75.00%.

:exclamation: Current head a0c4066 differs from pull request most recent head d65d8fa. Consider uploading reports for the commit d65d8fa to get more accurate results

@@                   Coverage Diff                    @@
##           release/devel_v6.0.0     #232      +/-   ##
========================================================
+ Coverage                 57.51%   57.58%   +0.07%     
========================================================
  Files                        42       42              
  Lines                      7720     7719       -1     
========================================================
+ Hits                       4440     4445       +5     
+ Misses                     3280     3274       -6     

Impacted Files	Coverage Δ
R/sw_Pedotransfer_Functions.R	69.15% <75.00%> (+1.32%)	:arrow_up: